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Balancing Chemical Equations

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Balancing Chemical Equations

A chemical plant is checking its reaction records before a practical. Balance each equation using the smallest whole-number coefficients. Never change a formula’s subscripts: coefficients change the number of particles; subscripts change the substance.

Spot the rules

Use atom counts and conservation of mass to audit the equations.

1.A trainee changes H₂O into H₂O₂ to make the oxygen count match in H₂ + O₂ → H₂O. Explain why this is not a valid way to balance the equation, and write the balanced equation instead.

2.Select the statement that best explains why a balanced equation obeys the law of conservation of mass.

It has the same number of molecules on each side.
It has the same number of atoms of every element on each side, so atoms are rearranged rather than created or destroyed.
The products always have less mass than the reactants.
It uses the same coefficient for every substance.

3.Balance the formation of water: __ H₂ + __ O₂ → __ H₂O. Then state the atom count for hydrogen and oxygen on each side.

4.Balance methane combustion: __ CH₄ + __ O₂ → __ CO₂ + __ H₂O. Show how your oxygen count checks out.

5.An equation has 3 oxygen atoms in Al₂O₃. How many oxygen atoms are represented by 2Al₂O₃? Choose one.

Balance the plant’s reaction log

Write the smallest whole-number coefficients. Include 1 if useful in your working, though it is usually omitted in the final equation.

6.A hydrogen fuel cell combines hydrogen and oxygen to form water. Balance: __ H₂ + __ O₂ → __ H₂O.

7.Nitrogen and hydrogen react to make ammonia. Balance: __ N₂ + __ H₂ → __ NH₃. Check nitrogen before hydrogen.

8.Iron reacts with oxygen to form iron(III) oxide. Balance: __ Fe + __ O₂ → __ Fe₂O₃.

9.Magnesium reacts with hydrochloric acid to form magnesium chloride and hydrogen. Write and balance the equation using formulas Mg, HCl, MgCl₂ and H₂.

10.Aluminium reacts with chlorine to form aluminium chloride. Balance: __ Al + __ Cl₂ → __ AlCl₃. Explain briefly why the number of chlorine atoms is even in the balanced equation.

Diagnose a faulty record

Use precise chemical reasoning—not just the final coefficients.

11.A student writes 2Na + Cl₂ → 2NaCl and claims it is unbalanced because there are two chlorine atoms on the left but only one on the right. Is the claim correct? Explain using the coefficient and subscript.

12.True or false: In a balanced equation, the total number of atoms must be identical on both sides, even when different elements are counted separately.

TrueFalse

13.The equation C₃H₈ + O₂ → CO₂ + H₂O describes propane combustion. Balance it and verify the counts of C, H and O atoms.